Method for the use of [11C] carbon monoxide in labeling synthesis of 11C-labelled esters by photo-induced free radical carbonylation

ABSTRACT

Methods and reagents for photo-initiated carbonylation with carbon-isotope labeled carbon monoxide using alkyl/aryl iodides with alcohols pretreated by a base are provided. The resultant carbon-isotope labeled esters are useful as radiopharmaceuticals, especially for use in Positron Emission Tomography (PET). Associated kits for PET studies are also provided.

FIELD OF THE INVENTION

The present invention relates to a method and an apparatus for the useof carbon-isotope monoxide in labeling synthesis. More specifically, theinvention relates to a method and apparatus for producing an [¹¹C]carbonmonoxide enriched gas mixture from an initial [¹¹C]carbon dioxide gasmixture, and using the produced gas mixture in labeling synthesis byphoto-initiated carbonylation. Radiolabeled esters are provided usingalkyl or aryl iodides and alcohols as precursors.

BACKGROUND OF THE INVENTION

Tracers labeled with short-lived positron emitting radionuclides (e.g.¹¹C, t_(1/2)=20.3 min) are frequently used in various non-invasive invivo studies in combination with positron emission tomography (PET).Because of the radioactivity, the short half-lives and the submicromolaramounts of the labeled substances, extraordinary synthetic proceduresare required for the production of these tracers. An important part ofthe elaboration of these procedures is development and handling of new¹¹C-labelled precursors. This is important not only for labeling newtypes of compounds, but also for increasing the possibility of labelinga given compound in different positions.

During the last two decades carbonylation chemistry using carbonmonoxide has developed significantly. The recent development of methodssuch as palladium-catalyzed carbonylative coupling reactions hasprovided a mild and efficient tool for the transformation of carbonmonoxide into different carbonyl compounds.

Carbonylation reactions using [¹¹C]carbon monoxide has a primary valuefor PET-tracer synthesis since biologically active substances oftencontain carbonyl groups or functionalities that can be derived from acarbonyl group. The syntheses are tolerant to most functional groups,which means that complex building blocks can be assembled in thecarbonylation step to yield the target compound. This is particularlyvaluable in PET-tracer synthesis where the unlabelled substrates shouldbe combined with the labeled precursor as late as possible in thereaction sequence, in order to decrease synthesis-time and thus optimizethe uncorrected radiochemical yield.

When compounds are labeled with ¹¹C, it is usually important to maximizespecific radioactivity. In order to achieve this, the isotopic dilutionand the synthesis time must be minimized. Isotopic dilution fromatmospheric carbon dioxide may be substantial when [¹¹C]carbon dioxideis used in a labeling reaction. Due to the low reactivity andatmospheric concentration of carbon monoxide (0.1 ppm vs. 3.4×10⁴ ppmfor CO₂), this problem is reduced with reactions using [¹¹C]carbonmonoxide.

The synthesis of [¹¹C]carbon monoxide from [¹¹C]carbon dioxide using aheated column containing reducing agents such as zinc, charcoal ormolybdenum has been described previously in several publications.Although [¹¹C]carbon monoxide was one of the first ¹¹C-labelledcompounds to be applied in tracer experiments in human, it has untilrecently not found any practical use in the production of PET-tracers.One reason for this is the low solubility and relative slow reactionrate of [¹¹C]carbon monoxide which causes low trapping efficiency inreaction media. The general procedure using precursors such as[¹¹C]methyl iodide, [¹¹C]hydrogen cyanide or [¹¹C]carbon dioxide is totransfer the radioactivity in a gas-phase, and trap the radioactivity byleading the gas stream through a reaction medium. Until recently thishas been the only accessible procedure to handle [¹¹C]carbon monoxide inlabeling synthesis. With this approach, the main part of the labelingsyntheses with [¹¹C]carbon monoxide can be expected to give a very lowyield or fail completely.

There are only a few examples of practically valuable ¹¹C-labellingsyntheses using high pressure techniques (>300 bar). In principal, highpressures can be utilized for increasing reaction rates and minimizingthe amounts of reagents. One problem with this approach is how toconfine the labeled precursor in a small high-pressure reactor. Anotherproblem is the construction of the reactor. If a common column type ofreactor is used (i.e. a cylinder with tubing attached to each end), thegas-phase will actually become efficiently excluded from the liquidphase at pressurization. The reason is that the gas-phase, in contractedform, will escape into the attached tubing and away from the bulk amountof the liquid reagent.

The cold-trap technique is widely used in the handling of ¹¹C-labelledprecursors, particularly in the case of [¹¹C]carbon dioxide. Theprocedure has, however, only been performed in one single step and thelabeled compound was always released in a continuous gas-streamsimultaneous with the heating of the cold-trap. Furthermore, the volumeof the material used to trap the labeled compound has been relativelarge in relation to the system to which the labeled compound has beentransferred. Thus, the option of using this technique for radicalconcentration of the labeled compound and miniaturization of synthesissystems has not been explored. This is especially noteworthy in view ofthe fact that the amount of a ¹¹C-labelled compound usually is in therange 20-60 nmol.

Recent technical development for the production and use of [¹¹C] carbonmonoxide has made this compound useful in labeling synthesis. WO02/102711 describes a system and a method for the production and use ofa carbon-isotope monoxide enriched gas-mixture from an initialcarbon-isotope dioxide gas mixture. [¹¹C] carbon monoxide may beobtained in high radiochemical yield from cyclotron produced [¹¹C]carbon dioxide and can be used to yield target compounds with highspecific radioactivity. This reactor overcomes the difficulties listedabove and is useful in synthesis of ¹¹C-labelled compounds using [¹¹C]carbon monoxide in palladium or selenium mediated reaction. With suchmethod, a broad array of carbonyl compounds can be labeled (Kilhlberg,T.; Langstrom, B. J., Org. Chem. 1999, 9201-9205). The use of transitionmetal mediated reactions is, however, restricted by problems related tothe competing β-hydride elimination reaction, which excludes or at leastseverely restricts utilization of organic electrophiles having hydrogenin β-position. Thus, a limitation of the transition metal mediatedreactions is that most alkyl halides could not be used as substrates dueto the 13-hydride elimination reaction. One way to circumvent thisproblem is to use free-radical chemistry based on light irradiation ofalkyl halides. We earlier succeeded in using free-radical chemistry forthe carbonylation of alkyl iodides using amines to yield labeled amides.However, the attempt to yield labeled esters in an analogous way (usingalcohols as a reactant instead of amines) is challenged by the lowreactivity of alcohols in these reaction conditions (typically theyields of esters compared to those of amides are lower by 10 to 100times). Therefore, there is a need for a method in order to usephoto-induced free radical carbonylation with weakly reacting alcoholsto circumvent the problem with β-hydride elimination to complement thepalladium mediated reactions and provide target structures with highyield to further increase the utility of [¹¹C] carbon monoxide inpreparing useful PET tracers.

Discussion or citation of a reference herein shall not be construed asan admission that such reference is prior art to the present invention.

SUMMARY OF THE INVENTION

The present invention provides a method for labeling synthesis,comprising:

(a) providing a UV reactor assembly comprising a high pressure reactionchamber, a UV spot light source with a light guide, wherein the lightguide is used to provide photo irradiation of a reaction mixture througha window in the reaction chamber,

(b) dissolving or reacting a base with an alcohol or a solution ofalcohol in another solvent,

(c) adding an alkyl or aryl iodides to the solution of step (b) to givea reagent volume to be labeled,

(d) introducing a carbon-isotope monoxide enriched gas-mixture into thereaction chamber of the UV reactor assembly via the gas inlet,

(e) introducing at high-pressure said reagent volume into the reactionchamber via the liquid inlet,

(f) turning on the UV spot light source and waiting a predetermined timewhile the labeling synthesis occur, and

(g) collecting labeled ester from the reaction chamber.

The present invention also provides a system for labeling synthesis,comprising: a UV reactor assembly comprising a high pressure reactionchamber, a UV spot light source with a light guide, wherein a lightguide is used to provide photo irradiation of the reaction mixturethrough a window in the reaction chamber thereof, wherein the photoirradiation from the light source, which stands at the distance from thereaction chamber, is delivered through the window of the reactionchamber.

The present invention further provides a method for the synthesis oflabeled esters using photo-initiated carbonylation with [¹¹C] carbonmonoxide using alcohols pretreated with a base and alkyl or aryliodides.

In yet another embodiment, the invention also provides [¹¹C]-labeledesters. In still another embodiment, the invention provides kits for useas PET tracers comprising [¹¹C]-labeled esters.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a flow chart over the method according to the invention

FIG. 2 is a schematic view of a carbon-isotope monoxide production andlabeling-system according to the invention.

FIG. 3 is the cross-sectional view of the reaction chamber.

FIG. 4 is a view of the UV spot light source.

FIG. 5 shows how the reaction chamber, magnetic stirrer, and the UV spotlight source are arranged into the UV reactor assembly.

FIG. 6 a and 6 b show alternative embodiments of a reaction chamberaccording to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The object of the invention is to provide a method and a system forproduction of and use of carbon-isotope monoxide in labeling synthesisthat overcomes the drawbacks of the prior art devices. This is achievedby the method and system claimed in the invention.

One advantage with such a method and system is that nearly quantitativeconversion of carbon-isotope monoxide into labeled products can beaccomplished.

There are several other advantages with the present method and system.The high-pressure technique makes it possible to use low boilingsolvents such as diethyl ether at high temperatures (e.g. 200° C.). Theuse of a closed system consisting of materials that prevents gasdiffusion, increases the stability of sensitive compounds and could beadvantageous also with respect to Good Manufacturing Practice (GMP).

Still other advantages are achieved in that the resulting labeledcompound is highly concentrated, and that the miniaturization of thesynthesis system facilitates automation, rapid synthesis andpurification, and optimization of specific radioactivity throughminimization of isotopic dilution.

Most important is the opening of completely new synthesis possibilities,as exemplified by the present invention.

Embodiments of the invention will now be described with reference to thefigures.

The term carbon-isotope that is used throughout this applicationpreferably refers to ¹¹C, but it should be understood that ¹¹C may besubstituted by other carbon-isotopes, such as ¹³C and ¹⁴C, if desired.

FIG. 1 shows a flow chart over the method according to the invention,which firstly comprises production of a carbon-isotope monoxide enrichedgas-mixture and secondly a labeling synthesis procedure. More in detailthe production part of the method comprises the steps of:

Providing carbon-isotope dioxide in a suitable carrier gas of a typethat will be described in detail below.

Converting carbon-isotope dioxide to carbon-isotope monoxide byintroducing said gas mixture in a reactor device which will be describedin detail below.

Removing traces of carbon-isotope dioxide by flooding the convertedgas-mixture through a carbon dioxide removal device whereincarbon-isotope dioxide is trapped but not carbon-isotope monoxide northe carrier gas. The carbon dioxide removal device will be described indetail below.

Trapping carbon-isotope monoxide in a carbon monoxide trapping device,wherein carbon-isotope monoxide is trapped but not said carrier gas. Thecarbon monoxide trapping device will be described in detail below.

Releasing said trapped carbon-isotope monoxide from said trappingdevice, whereby a volume of carbon-isotope monoxide enriched gas-mixtureis achieved.

The production step may further comprise a step of changing carrier gasfor the initial carbon-isotope dioxide gas mixture if the initialcarbon-isotope dioxide gas mixture is comprised of carbon-isotopedioxide and a first carrier gas not suitable as carrier gas for carbonmonoxide due to similar molecular properties or the like, such asnitrogen. More in detail the step of providing carbon-isotope dioxide ina suitable second carrier gas such as He, Ar, comprises the steps of:

Flooding the initial carbon-isotope dioxide gas mixture through a carbondioxide trapping device, wherein carbon-isotope dioxide is trapped butnot said first carrier gas. The carbon dioxide trapping device will bedescribed in detail below.

Flushing said carbon dioxide trapping device with said suitable secondcarrier gas to remove the remainders of said first carrier gas.

Releasing said trapped carbon-isotope dioxide in said suitable secondcarrier gas.

The labeling synthesis step that may follow the production step utilizesthe produced carbon-isotope dioxide enriched gas-mixture as labelingreactant. More in detail the step of labeling synthesis comprises thesteps of:

Providing a UV reactor assembly comprising a UV spot light source and ahigh pressure reaction chamber having a liquid reagent inlet and alabeling reactant inlet in a bottom surface thereof. In a preferredembodiment, the UV reactor assembly further comprises a magnetic stirrerand a magnetic stirring bar. In another preferred embodiment, the UVreactor assembly further comprises a protective housing and a benchwhere the reaction chamber, UV spot light guide and the magnetic stirrercan be mounted. The UV reactor assembly and the reaction chamber will bedescribed in detail below.

Providing a reagent volume that is to be labeled. The reagent volume canbe prepared in following steps: 1. Dissolve or react a base with analcohol or a solution of an alcohol in another solvent; 2. Add alkyl oraryl iodide to the solution of step 1 to form a reagent volume as lateas possible before being introduced into the high pressure reactionchamber. Definition and examples of base will be provided below.

Introducing the carbon-isotope monoxide enriched gas-mixture into thereaction chamber via the labeling reactant inlet.

Introducing, at high pressure, said liquid reagent into the reactionchamber via the liquid reagent inlet.

Turning on the UV spot light source and waiting a predetermined timewhile the labeling synthesis occurs.

Collecting the solution of labeled ester from the reaction chamber.

The step of waiting a predetermined time may further comprise adjustingthe temperature of the reaction chamber such that the labeling synthesisis enhanced.

FIG. 2 schematically shows a [¹¹C]carbon dioxide production andlabeling-system according to the present invention. The system iscomprised of three main blocks, each handling one of the three mainsteps of the method of production and labeling:

-   -   Block A is used to perform a change of carrier gas for an        initial carbon-isotope dioxide gas mixture, if the initial        carbon-isotope dioxide gas mixture is comprised of        carbon-isotope dioxide and a first carrier gas not suitable as        carrier gas for carbon monoxide.    -   Block B is used to perform the conversion from carbon-isotope        dioxide to carbon-isotope monoxide, and purify and concentrate        the converted carbon-isotope monoxide gas mixture.    -   Block C is used to perform the carbon-isotope monoxide labeling        synthesis.

Block A is normally needed due to the fact that carbon-isotope dioxideusually is produced using the 14N(p,α)¹¹C reaction in a target gascontaining nitrogen and 0.1% oxygen, bombarded with 17 MeV protons,whereby the initial carbon-isotope dioxide gas mixture comprisesnitrogen as carrier gas. However, compared with carbon monoxide,nitrogen show certain similarities in molecular properties that makes itdifficult to separate them from each other, e.g. in a trapping device orthe like, whereby it is difficult to increase the concentration ofcarbon-isotope monoxide in such a gas mixture. Suitable carrier gasesmay instead be helium, argon or the like. Block A can also used tochange the pressure of the carrier gas (e.g. from 1 to 4 bar), in casethe external system does not tolerate the gas pressure needed in block Band C. In an alternative embodiment the initial carbon-isotope dioxidegas mixture is comprised of carbon-isotope dioxide and a first carriergas that is well suited as carrier gas for carbon monoxide, whereby theblock A may be simplified or even excluded.

According to a preferred embodiment (FIG. 2), block A is comprised of afirst valve V1, a carbon dioxide trapping device 8, and a second valveV2.

The first valve V1 has a carbon dioxide inlet 10 connected to a sourceof initial carbon-isotope dioxide gas mixture 12, a carrier gas inlet 14connected to a source of suitable carrier gas 16, such as helium, argonand the like. The first valve VI further has a first outlet 18 connectedto a first inlet 20 of the second valve V2, and a second outlet 22connected to the carbon dioxide trapping device 8. The valve V1 may beoperated in two modes A, B, in mode A the carbon dioxide inlet 10 isconnected to the first outlet 18 and the carrier gas inlet 14 isconnected to the second outlet 22, and in mode B the carbon dioxideinlet 10 is connected to the second outlet 22 and the carrier gas inlet14 is connected to the first outlet 18.

In addition to the first inlet 20, the second valve V2 has a secondinlet 24 connected to the carbon dioxide trapping device 8. The secondvalve V2 further has a waste outlet 26, and a product outlet 28connected to a product inlet 30 of block B. The valve V2 may be operatedin two modes A, B, in mode A the first inlet 20 is connected to thewaste outlet 26 and the second inlet 24 is connected to the productoutlet 28, and in mode B the first inlet 20 is connected to the productoutlet 28 and the second inlet 24 is connected to the waste outlet 26.

The carbon dioxide trapping device 8 is a device wherein carbon dioxideis trapped but not said first carrier gas, which trapped carbon dioxidethereafter may be released in a controlled manner. This may preferablybe achieved by using a cold trap, such as a column containing a materialwhich in a cold state, (e.g. −196° C. as in liquid nitrogen or −186° C.as in liquid argon) selectively trap carbon dioxide and in a warm state(e.g. +50° C.) releases the trapped carbon dioxide. (In this text theexpression “cold trap” is not restricted to the use of cryogenics. Thus,materials that trap the topical compound at room temperature and releaseit at a higher temperature are included). One suitable material isporapac Q®. The trapping behavior of a porapac-column is related todipole-dipole interactions or possibly Van der Waal interaktions. Thesaid column 8 is preferably formed such that the volume of the trappingmaterial is to be large enough to efficiently trap (>95%) thecarbon-isotope dioxide, and small enough not to prolong the transfer oftrapped carbon dioxide to block B. In the case of porapac Q® and a flowof 100 ml nitrogen/min, the volume should be 50-150 μl. The cooling andheating of the carbon dioxide trapping device 8 may further be arrangedsuch that it is performed as an automated process, e.g. by automaticallylowering the column into liquid nitrogen and moving it from there into aheating arrangement.

According to the preferred embodiment of FIG. 2 block B is comprised ofa reactor device 32 in which carbon-isotope dioxide is converted tocarbon-isotope monoxide, a carbon dioxide removal device 34, acheck-valve 36, and a carbon monoxide trapping device 38, which all areconnected in a line.

In the preferred embodiment the reactor device 32 is a reactor furnacecomprising a material that when heated to the right temperature intervalconverts carbon-isotope dioxide to carbon-isotope monoxide. A broadrange of different materials with the ability to convert carbon dioxideinto carbon monoxide may be used, e.g. zinc or molybdenum or any otherelement or compound with similar reductive properties. If the reactordevice 32 is a zinc furnace it should be heated to 400° C., and it isimportant that the temperature is regulated with high precision. Themelting point of zinc is 420° C. and the zinc-furnace quickly loses itability to transform carbon dioxide into carbon monoxide when thetemperature reaches over 410° C., probably due to changed surfaceproperties. The material should be efficient in relation to its amountto ensure that a small amount can be used, which will minimize the timeneeded to transfer radioactivity from the carbon dioxide trapping device8 to the subsequent carbon monoxide trapping device 38. The amount ofmaterial in the furnace should be large enough to ensure a practicallife-time for the furnace (at least several days). In the case of zincgranulates, the volume should be 100-1000 μl.

The carbon dioxide removal device 34 is used to remove traces ofcarbon-isotope dioxide from the gas mixture exiting the reactor device32. In the carbon dioxide removal device 34, carbon-isotope dioxide istrapped but not carbon-isotope monoxide nor the carrier gas. The carbondioxide removal device 34 may be comprised of a column containingascarite® (i.e. sodium hydroxide on silica). Carbon-isotope dioxide thathas not reacted in the reactor device 32 is trapped in this column (itreacts with sodium hydroxide and turns into sodium carbonate), whilecarbon-isotope monoxide passes through. The radioactivity in the carbondioxide removal device 34 is monitored as a high value indicates thatthe reactor device 32 is not functioning properly.

Like the carbon dioxide trapping device 8, the carbon monoxide trappingdevice 38, has a trapping and a releasing state. In the trapping statecarbon-isotopemonoxide is selectively trapped but not said carrier gas,and in the releasing state said trapped carbon-isotope monoxide isreleased in a controlled manner. This may preferably be achieved byusing a cold trap, such as a column containing silica which selectivelytrap carbon monoxide in a cold state below −100° C., e.g. −196° C. as inliquid nitrogen or −186° C. as in liquid argon, and releases the trappedcarbon monoxide in a warm state (e.g. +50° C.). Like the porapac-column,the trapping behavior of the silica-column is related to dipole-dipoleinteractions or possibly Van der Waal interactions. The ability of thesilica-column to trap carbon-isotope monoxide is reduced if the helium,carrying the radioactivity, contains nitrogen. A rationale is that sincethe physical properties of nitrogen are similar to carbon monoxide,nitrogen competes with carbon monoxide for the trapping sites on thesilica.

According to the preferred embodiment of FIG. 2, block C is comprised ofa first and a second reaction chamber valve V3 and V4, a reagent valveV5, an injection loop 70 and a solvent valve V6, and the UV reactorassembly 51 which comprises a UV lamp 91, a concave mirror 92 and areaction chamber 50.

The first reaction chamber valve V3 has a gas mixture inlet 40 connectedto the carbon monoxide trapping device 38, a stop position 42, acollection outlet 44, a waste outlet 46, and a reaction chamberconnection port 48 connected to a gas inlet 52 of the reaction chamber50. The first reaction chamber valve V3 has four modes of operation A toD. The reaction chamber connection port 48 is: in mode A connected tothe gas mixture inlet 40, in mode B connected to the stop position 42,in mode C connected to the collection outlet 44, and in mode D connectedto the waste outlet 46.

FIG. 3 shows the reaction chamber 50 (micro-autoclave) which has a gasinlet 52 and a liquid inlet 54, which are arranged such that theyterminate at the bottom surface of the chamber. Gas inlet 52 may also beused as product outlet after the labeling is finished. During operationthe carbon-isotope monoxide enriched gas mixture is introduced into thereaction chamber 50 through the gas inlet 52, where after the liquidreagent at high pressure enters the reaction chamber 50 through theliquid inlet 54. FIG. 6 a and 6 b shows schematic views of two preferredreaction chambers 50 in cross section. FIG. 6 a is a cylindrical chamberwhich is fairly easy to produce, whereas the spherical chamber of FIG. 6b is the most preferred embodiment, as the surface area to volume-ratioof the chamber is further minimized. A minimal surface area tovolume-ratio optimizes the recovery of labeled product and minimizespossible reactions with the surface material. Due to the “diving-bellconstruction” of the reaction chamber 50, both the gas inlet 52 and theliquid inlet 54 becomes liquid-filled and the reaction chamber 50 isfilled from the bottom upwards. The gas-volume containing thecarbon-isotope monoxide is thus trapped and given efficient contact withthe reaction mixture. Since the final pressure of the liquid isapproximately 80 times higher than the original gas pressure, the finalgas volume will be less than 2% of the liquid volume according to thegeneral gas-law. Thus, a pseudo one-phase system will result. In theinstant application, the term “pseudo one-phase system” means a closedvolume with a small surface area to volume-ratio containing >96% liquidand <4% gas at pressures exceeding 200 bar. In most syntheses thetransfer of carbon monoxide from the gas-phase to the liquid phase willprobably not be the rate limiting step. After the labeling is finishedthe labeled volume is nearly quantitatively transferred from thereaction chamber by the internal pressure via the gas inlet/productoutlet 52 and the first reaction chamber valve V3 in position C.

In a specific embodiment, FIG. 3 shows a reaction chamber made fromstainless steel (Valco™) column end fitting 101. It is equipped withsapphire window 102, which is a hard material transparent to shortwavelength UV radiation. The window is pressed between two Teflonwashers 103 inside the drilled column end fitting to make the reactortight at high pressures. Temperature measurement can be accomplishedwith the thermocouple 104 attached by solder drop 105 to the outer sideof the reactor. A magnet stirrer (not shown) drives small Teflon coatedmagnet stirring bar 106 placed inside the reaction chamber. The magneticstirrer can be attached against the bottom of the reaction chamber.Distance between the magnet stirrer and the reactor should be minimal.

FIG. 4 shows a commercial UV spot light source 110 (for example,Hamamatsu Lightningcure™ LC5), which is an example of UV spot lightsources that can be used in the instant invention. Light source 110 hasnecessary means of operating and controlling the photo irradiation thatis produced, of the light source is available from the manufacturer(Hamamatsu Photonics K.K.). Thus intensity and time duration of thephoto irradiation are easily adjusted by an operator. Light source 110may be externally controlled by a computer, providing a possibility forautomating the reactor assembly. The photo irradiation is delivered tothe reaction vessel through a flexible light guide, which is anaccessory of Hamamatsu Lightningcure™ LC5. Thus light source 110 may beplaced at the distance from the reaction chamber providing thepossibility to save precious space inside a sheltered hot-cell, wherethe radiolabeling syntheses are carried out. Light source 110 complieswith the existing industrial safety standards. Further, optionalaccessories (e.g. changeable lamps, optical filters) are provided whichmay be advantageously used for adjusting the properties of the photoirradiation.

FIG. 5 shows the reaction chamber 50 situated a magnetic stirrer 201,with gas inlet/product outlet 52 and liquid inlet 54 facing the magneticstirrer 201. Top of the reaction chamber 50 is connected through theflexible light guide 202 to the UV spot light source (not shown).

Referring back to FIG. 2, the second reaction chamber valve V4 has areaction chamber connection port 56, a waste outlet 58, and a reagentinlet 60. The second reaction chamber valve V4 has two modes ofoperation A and B. The reaction chamber connection port 56 is: in mode Aconnected to the waste outlet 58, and in mode B it is connected to thereagent inlet 60.

The reagent valve V5, has a reagent outlet 62 connected to the reagentinlet 60 of the second reaction chamber valve V4, an injection loopinlet 64 and outlet 66 between which the injection loop 70 is connected,a waste outlet 68, a reagent inlet 71 connected to a reagent source, anda solvent inlet 72. The reagent valve V5, has two modes of operation Aand B. In mode A the reagent inlet 71 is connected to the injection loopinlet 64, and the injection loop outlet 66 is connected to the wasteoutlet 68, whereby a reagent may be fed into the injection loop 70. Inmode B the solvent inlet 72 is connected to the injection loop inlet 64,and the injection loop outlet 66 is connected to the reagent outlet 62,whereby reagent stored in the injection loop 70 may be forced via thesecond reaction chamber valve V4 into the reaction chamber 50 if a highpressure is applied on the solvent inlet 72.

The solvent valve V6, has a solvent outlet 74 connected to the solventinlet 72 of the reagent valve V5, a stop position 76, a waste outlet 78,and a solvent inlet 80 connected to a solvent supplying HPLC-pump (HighPerformance Liquid Chromatography) or any liquid-pump capable of pumpingorganic solvents at 0-10 ml/min at pressures up to 400 bar (not shown).The solvent valve V6, has two modes of operation A and B. In mode A thesolvent outlet 74 is connected to the stop position 76, and the solventinlet 80 is connected to the waste outlet 78. In mode B the solventoutlet 74 is connected to the solvent inlet 80, whereby solvent may bepumped into the system at high pressure by the HPLC-pump.

Except for the small volume of silica in the carbon monoxide trappingdevise 38, an important difference in comparison to the carbon dioxidetrapping device 8, as well as to all related prior art, is the procedureused for releasing the carbon monoxide. After the trapping of carbonmonoxide on carbon monoxide trapping devise 8, valve V3 is changed fromposition A to B to stop the flow from the carbon monoxide trappingdevise 38 and increase the gas-pressure on the carbon monoxide trappingdevise 38 to the set feeding gas pressure (3-5 bar). The carbon monoxidetrapping devise 38 is then heated to release the carbon monoxide fromthe silica surface while not significantly expanding the volume ofcarbon monoxide in the carrier gas. Valve V4 is changed from position Ato B and valve V3 is then changed from position B to A. At this instancethe carbon monoxide is rapidly and almost quantitatively transferred ina well-defined micro-plug into the reaction chamber 50. Micro-plug isdefined as a gas volume less than 10% of the volume of the reactionchamber 50, containing the topical substance (e.g. 1-20 μL). This uniquemethod for efficient mass-transfer to a small reaction chamber 50,having a closed outlet, has the following prerequisites:

-   -   A micro-column 38 defined as follows should be used. The volume        of the trapping material (e.g. silica) should be large enough to        efficiently trap (>95%) the carbon-isotope monoxide, and small        enough (<1% of the volume of a subsequent reaction chamber 50)        to allow maximal concentration of the carbon-isotope monoxide.        In the case of silica and a reaction chamber 50 volume of 200        μl, the silica volume should be 0.1-2 μl.    -   The dead volumes of the tubing and valve(s) connecting the        silica column and the reaction chamber 50 should be minimal        (<10% of the micro-autoclave volume).    -   The pressure of the carrier gas should be 3-5 times higher than        the pressure in the reaction chamber 50 before transfer (1        atm.).

In one specific preferred embodiment specifications, materials andcomponents are chosen as follows. High pressure valves from Valco®,Reodyne® or Cheminert® are used. Stainless steel tubing with o.d. 1/16″is used except for the connections to the porapac-column 8, thesilica-column 38 and the reaction chamber 50 where stainless steeltubing with o.d. 1/32″ are used in order to facilitate the translationmovement. The connections between V1, V2 and V3 should have an innerdiameter of 0.2-1 mm. The requirement is that the inner diameter shouldbe large enough not to obstruct the possibility to achieve the optimalflow of He (2-50 ml/min) through the system, and small enough not toprolong the time needed to transfer the radioactivity from theporapac-column 8 to the silica-column 38. The dead volume of theconnection between V3 and the autoclave should be minimized (<10% of theautoclave volume). The inner diameter (0.05-0.1 mm) of the connectionmust be large enough to allow optimal He flow (2-50 ml/min). The deadvolume of the connection between V4 and V5 should be less than 10% ofthe autoclave volume.

The porapac-column 8 preferably is comprised of a stainless steel tube(o.d.=⅛, i.d.=2 mm, 1=20 mm) filled with Porapac Q® and fitted withstainless steel screens. The silica-column 38 preferably is comprised ofa stainless steel tube (o.d= 1/16″, i.d.=0.1 mm) with a cavity (d=1 mm,h=1 mm, V=0.8 μl) in the end. The cavity is filled with silica powder(100/80 mesh) of GC-stationary phase type. The end of the column isfitted against a stainless steel screen.

It should be noted that a broad range of different materials could beused in the trapping devices. If a GC-material is chosen, the criterionsshould be good retardation and good peak-shape for carbon dioxide andcarbon monoxide respectively. The latter will ensure optimal recovery ofthe radioactivity.

Below a detailed description is given of a method of producingcarbon-isotope using an exemplary system as described above.

Preparations of the system are performed by the steps 1 to 5:

-   -   1. V1 in position A, V2 in position A, V3 in position A, V4 in        position A, helium flow on with a max pressure of 5 bar. With        this setting, the helium flow goes through the porapac column,        the zinc furnace, the silica column, the reaction chamber 50 and        out through V4. The system is conditioned, the reaction chamber        50 is rid of solvent and it can be checked that helium can be        flowed through the system with at least 10 ml/min. UV lamp 91 is        turned on.    -   2. The zinc-furnace is turned on and set at 400° C.    -   3. The porapac and silica-columns are cooled with liquid        nitrogen. At −196° C., the porapac and silica-column efficiently        traps carbon-isotope dioxide and carbon-isotope monoxide        respectively.    -   4. V5 in position A (load). The injection loop (250 μl),        attached to V5, is loaded with the reaction mixture.    -   5. The HPLC-pump is attached to a flask with freshly distilled        THF (or other high quality solvent) and primed. V6 in position        A.

Production of carbon-isotope dioxide may be performed by the steps 6 to7:

-   -   6. Carbon-isotope dioxide is produced using the 14N(p,α)¹¹C        reaction in a target gas containing nitrogen (AGA, Nitrogen 6.0)        and 0.1% oxygen (AGA. Oxygen 4.8), bombarded with 17 MeV        protons.    -   7. The carbon-isotope dioxide is transferred to the apparatus        using nitrogen with a flow of 100 ml/min.

Synthesis of carbon-isotope may thereafter be performed by the steps 8to 16

-   -   8. V1 in position B and V2 in position B. The nitrogen flow        containing the carbon-isotope dioxide is now directed through        the porapac-column (cooled to −196° C.) and out through a waste        line. The radioactivity trapped in the porapac-column is        monitored.    -   9. When the radioactivity has peaked, V1 is changed to        position A. Now a helium flow is directed through the        porapac-column and out through the waste line. By this operation        the tubings and the porapac-column are rid of nitrogen.    -   10. V2 in position A and the porapac-column is warmed to about        50° C. The radioactivity is now released from the porapac-column        and transferred with a helium flow of 10 ml/min into the        zinc-furnace where it is transformed into carbon-isotope        monoxide.    -   11. Before reaching the silica-column (cooled to −196° C.), the        gas flow passes the ascarite-column. The carbon-isotope monoxide        is now trapped on the silica-column. The radioactivity in the        silica-column is monitored and when the value has peaked, V3 is        set to position B and then V4 is set to position B.    -   12. The silica-column is heated to approximately 50° C., which        releases the carbon-isotope monoxide. V3 is set to position A        and the carbon-isotope monoxide is transferred to the reaction        chamber 50 within 15 s.    -   13. V3 is set to position B, V5 is set to position B, the        HPLC-pump is turned on (flow 7 ml/min) and V6 is set to        position B. Using the pressurised THF (or other solvent), the        reaction mixture is transferred to the reaction chamber 50.

When the HPLC-pump has reached its set pressure limit (e.g 40 Mpa), itis automatically turned off and then V6 is set to position A.

-   -   14. UV spot light source 110, magnetic stirrer 201 and magnet        stirring bar 106 in reaction chamber 50 are turned on.    -   15. After a sufficient reaction-time (usually 5 min), V3 is set        to position C and the content of the reaction chamber 50 is        transferred to a collection vial.    -   16. The reaction chamber 50 can be rinsed by the following        procedure: V3 is set to position B, the HPLC-pump is turned on,        V6 is set to position B and when maximal pressure is reached V6        is set to position A and V3 is set to position 3 thereby        transferring the rinse volume to the collection vial.

With the recently developed fully automated version of the reactionchamber 50 system according to the invention, the value of [¹¹C]carbonmonoxide as a precursor for ¹¹C-labelled tracers has become comparablewith [¹¹C]methyl iodide. Currently, [¹¹C]methyl iodide is the mostfrequently used ¹¹C-precursor due to ease in production and handling andsince groups suitable for labeling with [¹¹C]methyl iodide (e.g. heteroatom bound methyl groups) are common among biologically activesubstances. Carbonyl groups, which can be conveniently labeled with[¹¹C]carbon monoxide, are also common among biologically activesubstances. In many cases, due to metabolic events in vivo, a carbonylgroup may even be more advantageous than a methyl group as labelingposition. The use of [¹¹C]carbon monoxide for production of PET-tracersmay thus become an interesting complement to [¹¹C]methyl iodide.Furthermore, through the use of similar technology, this method willmost likely be applicable for synthesis of ¹³C and ¹⁴C substitutedcompounds.

The main advantage of the present invention is to overcome thelimitations of transition metal-mediated reaction to synthesize¹¹C-labeled esters using alkyl/aryl iodides and alcohols as precursors.The levels of specific radioactivity are high compared with alternativemethods such as the use of Grignard reactions for preparation of[carbonyl-¹¹C] esters. Iodides used in this invention have a formula RI,where R is linear or cyclic alkyl or substituted alkyl, aryl orsubstituted aryl, and may contain chloro, fluoro, ester and carboxylgroups, which are separated by at least one carbon atom from the carbonatom bearing the iodide atom. Alcohol used may be a primary, secondaryor tertiary alcohol, or phenol. A base is defined as any organic orinorganic compound that produces RO⁻ anion upon the reaction withalcohols (but does not produce any other products which may be reactivetowards reagents, intermediates, and products that will hinder thedesired radiolabelling transformation). Examples of base include alkalimetal hydrides (for example, KH, NaH), hydroxides (for example, KOH),carbonates (for example, K₂CO₃, Cs₂CO₃), alkali metal amides (forexample, lithium hexamethyldisilylamide), alkyl or aryl metals (forexample, butyl lithium, phenyl lithium). Alcohols are pretreated by sucha base before mixing with RI. The term “pretreat” is meant such a basedissolves in or reacts with alcohols. Alternatively solutions ofalkoxides, which in many cases are commercially available, may be used.The resultant labeled esters have a formula

wherein R is defined as above. They provide valuable PET tracers invarious PET studies. In an embodiment of the present invention, itprovides kits for use as PET tracers comprising [¹¹C]-labeled esters.

Such kits are designed to give sterile products suitable for humanadministration, e.g. direct injection into the bloodstream. Suitablekits comprise containers (e.g. septum-sealed vials) containing the[¹¹C]-labeled esters.

The kits may optionally further comprise additional components such asradioprotectant, antimicrobial preservative, pH-adjusting agent orfiller.

By the term “radioprotectant” is meant a compound which inhibitsdegradation reactions, such as redox processes, by trappinghighly-reactive free radicals, such as oxygen-containing free radicalsarising from the radiolysis of water. The radioprotectants of thepresent invention are suitably chosen from: ascorbic acid,para-aminobenzoic acid (i.e. 4-aminobenzoic acid), gentisic acid (i.e.2,5-dihydroxybenzoic acid) and salts thereof.

By the term “antimicrobial preservative” is meant an agent whichinhibits the growth of potentially harmful micro-organisms such asbacteria, yeasts or moulds. The antimicrobial preservative may alsoexhibit some bactericidal properties, depending on the dose. The mainrole of the antimicrobial preservative(s) of the present invention is toinhibit the growth of any such micro-organism in the pharmaceuticalcomposition post-reconstitution, i.e. in the radioactive diagnosticproduct itself. The antimicrobial preservative may, however, alsooptionally be used to inhibit the growth of potentially harmfulmicro-organisms in one or more components of the kit of the presentinvention prior to reconstitution. Suitable antimicrobial preservativesinclude: the parabens, i.e., ethyl, propyl or butyl paraben or mixturesthereof; benzyl alcohol; phenol; cresol; cetrimide and thiomersal.Preferred antimicrobial preservative(s) are the parabens.

The term “pH-adjusting agent” means a compound or mixture of compoundsuseful to ensure that the pH of the reconstituted kit is withinacceptable limits (approximately pH 4.0 to 10.5) for humanadministration. Suitable such pH-adjusting agents includepharmaceutically acceptable buffers, such as tricine, phosphate or TRIS[i.e. tris(hydroxymethyl)aminomethane], and pharmaceutically acceptablebases such as sodium carbonate, sodium bicarbonate or mixtures thereof.When the ligand conjugate is employed in acid salt form, thepH-adjusting agent may optionally be provided in a separate vial orcontainer, so that the user of the kit can adjust the pH as part of amulti-step procedure.

By the term “filler” is meant a pharmaceutically acceptable bulkingagent which may facilitate material handling during production andlyophilisation. Suitable fillers include inorganic salts such as sodiumchloride, and water soluble sugars or sugar alcohols such as sucrose,maltose, mannitol or trehalose.

General reaction scheme for the synthesis of labeled esters are asillustrated below:

wherein R is as defined above. * indicates the ¹¹C-labeled position. Inalternative embodiments, an alcohol may be the reactant in the finalstep instead of an alkoxide.

EXAMPLES

The invention is further described in the following examples which arein no way intended to limit the scope of the invention.

Example 1 Precursors and Resultant Products

Precursors that were used to label esters are shown in List. 1.

List 1a. Iodides used as precursors in the synthesis of labeled esters

List 1b. Alcohols used as precursors in the synthesis of labeled esters

The following experiments illustrate the present invention. Radicalcarbonylation using submicromolar amounts of [¹¹C]carbon monoxide isperformed yielding labeled with the esters shown in Table 1 as targetcompounds. Table 1 Radiochemical yields for ¹¹C-labelled esters Table 1Radiochemical yields for ¹¹C-labelled esters Additive ¹¹CO PurityYield^(b) Isolated n Labelled compound^(a) Solvent (mmol) conv. (%) (%)(%) Yield (%) 1

THF THF LiHDMS (0.1) — 84 ± 1 19 97 ± 1 10 81 ± 3  2 67 ± 1 — 2

Me₂CHOH KOH (0.05) 87 86 75 3

MeOH Me₂CO/MeOH(1:1) —— 42 ± 1 86 ± 1 93 ± 2 85 ± 1 39 ± 2 72 ± 1 33 ± 156 ± 1 4

THF BuLi (0.1) 80 ± 2 73 ± 2 58 ± 3 42 ± 3 5

THF/H₂O (4:1) THF KOH (0.1) BuLi (0.1) 99 69 56 82 55 57 38 49 6

THF/H₂O (4:1) THF/CH₃OH (5:2) KOH (0.1) BuLi (0.1) 82 78 84 83 69 65 6158 7

THF/CH₃OH (5:1) LiHDMS (0.02) 77 78 87 87 67 68 50 51 8

CH₃OH KOH (0.1) 84 74 62 57 9

THF/C₂H₅OH (4:1) LiHDMS (0.05) 84 ± 1 93 ± 3 78 ± 2 53 ± 3^(a)The position of ¹¹C label is denoted by an asterisk.^(b)Decay-corrected radiochemical yield determined by LC.^(c)Number of runs.^(d)Unseparated peaks.

Example 2 Experimental Setup

[¹¹C]Carbon dioxide production was performed using a Scanditronix MC-17cyclotron at Uppsala Imanet. The ¹⁴N(p,α)¹¹C reaction was employed in agas target containing nitrogen (Nitrogen 6.0) and 0.1% oxygen (Oxygen4.8), that was bombarded with 17 MeV protons.

[¹¹C]Carbon monoxide was obtained by reduction of [¹¹C]carbon dioxide asdescribed previously (Kihlberg, T.; Långström, B. Method and apparatusfor production and use of [ ¹¹ C] carbon monoxide in labeling synthesis.Swedish Pending Patent Application No. 0102174-0).

Liquid chromatographic analysis (LC) was performed with a gradient pumpand a variable wavelength UV-detector in series with a β⁺-flow detector.An automated synthesis apparatus, Synthia (Bjurling, P.; Reineck, R.;Westerberg, G.; Gee, A. D.; Sutcliffe, J.; Långström, B. In Proceedingsof the VIth workshop on targetry and target chemistry; TRIUMF:Vancouver, Canada, 1995; pp 282-284) was used for LC purification of thelabelled products.

Radioactivity was measured in an ion chamber. Xenon-mercury lamp wasused as a photo-irradiation source.

In the analysis of the ¹¹C-labeled compounds, isotopically unchangedreference substances were used for comparison in all the LC runs.

NMR spectra were recorded at 400 MHz for ¹H and at 100 MHz for ¹³C, at25° C. Chemical shifts were referenced to TMS via the solvent signals.

LC-MS analysis was performed with electrospray ionization.

Solvents: THF was distilled under nitrogen from sodium/benzophenone; allother solvents were commercial grade. The solvents were purged withhelium.

Alkyl iodides were commercially available or otherwise prepared fromcommercial alkyl bromides by the Finkelstein reaction.

Example 3 Preparation of [carbonyl-¹¹C] Esters

General procedure: An alcohol (200 μmol) and appropriate base (Table 1)was placed in a capped vial (1 mL, flushed beforehand with nitrogen toremove air) and dissolved in THF (500 μL); in some cases the alcohol wasused as a solvent instead of THF. An iodide (100 μmol) was added to thesolution ca. 7 min before the start of the synthesis. The resultingmixture was pressurized (over 40 MPa) into the autoclave, pre-chargedwith [¹¹C]carbon monoxide (10⁻⁸-10⁻⁹ mol) and helium gas mixture. Themixture was irradiated with the Xe—Hg lamp for 6 min with stirring at35° C. The crude reaction mixture was then transferred from theautoclave to a capped vial, held under reduced pressure. Aftermeasurement of the radioactivity the vial was purged with nitrogen andthe radioactivity was measured again. The crude product was diluted withacetonitrile or methanol (0.6 mL) and injected on the semi-preparativeLC. Analytical LC and LC-MS were used to assess the identity andradiochemical purity of the collected fractions.

Specific Embodiments, Citation of References

The present invention is not to be limited in scope by specificembodiments described herein. Indeed, various modifications of theinventions in addition to those described herein will become apparent tothese skilled in the art from the foregoing description and accompanyingfigures. Such modifications are intended to fall within the scope of theappended claims.

Various publications and patent applications are cited herein, thedisclosures of which are incorporated by reference in their entireties.

1. A method for labeling synthesis, comprising: (a) Providing a UVreactor assembly comprising a high pressure reaction chamber, a UV spotlight source with a light guide, wherein the light guide is used toprovide photo irradiation of a reaction mixture through a window in thereaction chamber, (b) Dissolving or reacting a base with alcohols or analcohol solution in another solvent, (c) adding alkyl or aryl iodides tothe solution of step (b) to give a reagent volume to be labeled, (d)introducing a carbon-isotope monoxide enriched gas-mixture into thereaction chamber of the UV reactor assembly via the gas inlet, (e)introducing at high pressure said reagent volume into the reactionchamber via the liquid inlet, (f) turning on the UV spot light sourceand waiting for a predetermined time while the labeling synthesis occur,and (g) collecting the labeled ester from the reaction chamber.
 2. Amethod of claim 1, wherein the carbon-isotope monoxide enrichedgas-mixture is produced by a method comprising: (a) providingcarbon-isotope dioxide in a suitable carrier gas, (b) convertingcarbon-isotope dioxide to carbon-isotope monoxide by introducing saidgas mixture in a reactor device, (c) trapping carbon-isotope monoxide ina carbon monoxide trapping device, wherein carbon-isotope monoxide istrapped but not said carrier gas, and (d) releasing said trappedcarbon-isotope monoxide from said trapping device in a well definedmicro-plug, whereby a volume of carbon-isotope monoxide enrichedgas-mixture is achieved.
 3. A method of claim 1, wherein thecarbon-isotope is ¹¹C, ¹³C, or ¹⁴C.
 4. A method of claim 1, wherein thecarbon-isotope is ¹¹C.
 5. A method of claim 1, wherein the step ofintroducing the reagent is performed using a pressure that is about 80times higher than the pressure before the introduction, in order tomaintain a pseudo one-phase system.
 6. A method of claim 1, wherein thestep of waiting a predetermined time comprises stirring in the reactionchamber to enhance the labeling synthesis.
 7. A method of claim 6,wherein the step of waiting a predetermined time further comprisesadjusting the temperature of the reaction chamber so that the labelingsynthesis is enhanced.
 8. A method for radiocarbonylation comprisingreaction of carbon-isotope monoxide with a compound of formula (I) andan alcohol of formula (II), which is pretreated with a base:RI   (I)R′OH   (II) to give a labeled ester of formula (III):

wherein R is linear or cyclic alkyl or substituted alkyl, aryl orsubstituted aryl and wherein the base is any organic or inorganiccompound that produces RO⁻ anion upon reacting with an alcohol or asolution of alcohol in another solvent.
 9. A method of claim 8, whereinR may contain chloro, fluro, ester, carboxyl groups, which is separatedby at least one carbon bearing the iodide atom.
 10. A method of claim 8,wherein the base comprises alkali metal hydrides, hydroxides,carbonates, alkali metal amides, alkyl or aryl metals.
 11. A method ofclaim 8, wherein the alcohol is a primary, secondary or tertiaryalcohol, or phenol.
 12. A method of claim 8, wherein the carbon-isotopemonoxide is [¹¹C]carbon monoxide.
 13. A carbon-isotope labeled compoundof formula (III),

wherein R is linear or cyclic alkyl or substituted alkyl, aryl orsubstituted aryl
 14. A carbon-isotope labeled compound of claim 13,wherein R may contain chloro, fluro, ester, carboxyl groups, which isseparated by at least one carbon bearing the iodide atom.
 15. A kit forPET study comprising a carbon-isotope labeled compound of formula (III),

wherein R is linear or cyclic alkyl or substituted alkyl, aryl orsubstituted aryl
 16. A kit of claim 15, further comprisingradioprotectant, antimicrobial preservative, pH-adjusting agent orfiller.
 17. A kit of claim 16, wherein the radiopretectant is selectedfrom ascorbic acid, para-aminobenzoic acid, gentisic acid and saltsthereof.
 18. A kit of claim 16, wherein the antimicrobial preservativeis selected from the parabens, benzyl alcohol, phenol, cresol, cetrimideand thiomersal.
 19. A kit of claim 16, wherein the pH-adjusting agent isa pharmaceutically acceptable buffer or a pharmaceutically acceptablebase, or mixtures thereof.
 20. A kit of claim 16, wherein the filler isinorganic salts, water soluble sugars or sugar alcohols.